This invention relates to an electrostatographic printing machine, and more particularly concerns an improved development system for use therein.
In the process of electrostatograhic printing, a latent image of an original document is formed and reproduced in viewable form on a copy sheet. The field of electrostatography includes electrophotography and electrography. In electrophotography, a photosensitive medium is employed to form, with the aid of electromagnetic radiation, an electrostatic latent image. Electrography utilizes an insulating medium to form, without the aid of electromagnetic radiation, a latent image. In both of the foregoing processes, the latent image is rendered visible by the process of development, i.e., depositing particles thereon. Frequently, the particles are then transferred from the latent image to a copy sheet, or, in some processes, the sheet having the latent image recorded thereon may serve also as a copy sheet. In either case, the resultant powder image deposited on the copy sheet is permanently affixed thereto by the process of applying heat an/or pressure. Hereinafter, an electrophotographic printing machine will be described as the illustrative embodiment of a printing machine incorporating the features of the present invention therein.
An electrophotographic printing machine employs a photoconductive member which is charged to sensitize the surface thereof. The charged photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive surface discharges the charge selectively in the irradiated areas, in accordance with the light intensity. This creates an electrostatic latent image on the photoconductive surface corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive surface, the latent image is eveloped by bringing a developer mix into contact therewith. Generally, a developer mix comprises dyed or colored heat-settable plastic powders, known in the art as toner particles, which are attracted triboelectrically to coarser carrier granules, such as ferromagnetic granules. The toner particles and carrier granules are selected such that the toner particles have the appropriate charge relative to the electrostatic latent image recorded on the photoconductive surface. In this way, when the developer mix is brought into contact with the latent image recorded on the photoconductive surface, the greater attractive force thereof causes the toner particles to be attracted from the carrier granules to the latent image. This general approach was originally disclosed by Carlson in U.S. Pat. No. 2,297,691 and has been further amplified and described by many related patents in the art.
In electrophotographic printing, the most commonly employed development systems are cascade systems and magnetic brush systems. In a cascade system, a conveyer is employed to move the developer mix in an upwardly direction, and then to release it over the electrostatic image. In this way, the developer mix cascades downwardly over the latent image. The toner particles are attracted from the carrier granules rendering the latent image visible. A cascade development apparatus may be speed limited in that gravity is employed to cause the developer mix to descend over the latent image. Contrawise, magnetic brush development systems are not necessarily speed limited. A magnetic brush development system generally employs carrier granules which are ferromagnetic in nature. These carrier granules are held to a developer roll by magnetic attraction. The carrier granules have the toner particles adhering thereto triboelectrically. The carrier granules and toner particles extend in an outwardly direction from the surface of the developer roll forming bristles which simulate a brush. This brush of carrier granules and toner particles is brought into contact with the latent image which attracts the toner particles thereto from the carrier granules. Many electrophotograhic printing machines employ this type of magnetic brush development system. However, when the process speed is increased, the developer mix experiences increased inertial forces as they travel around the curved path which brings them into the development zone. This may cause the carrier granules to impact the photoconductive surface with excessive force. Moreover, the centrifugal force exceeds the centripetal force provided by the magnetic force attracting the developer mix thereto. The excessive force with which the developer mix strikes the photoconductive surface can produce high background densities and increased impactation of the toner particles on the carrier granules. Furthermore, if the centrifugal force becomes too excessive, the carrier granules escape the restraining magnetic force field and the developer mix can be thrown or dumped from the magnetic brush developer roll. Problems of this type may be overcome by reducing the roller speed. However, the desired developer mix flow rate cannot then be attained and the copy density becomes too low.
Hereinbefore, various techniques have been devised to overcome the foregoing problems. These approaches primarily involve varying the spacing and speed of the developer rollers. Exemplary patents disclosing the foregoing are U.S. Pat. No. 3,543,720 issued to Drexler et al. in 1970, U.S. Pat. No. 3,703,395 issued to Drexler et al. in 1972, and U.S. Pat. No. 3,133,834 issued to Sowiak in 1964. The Sowiak patent discloses a photoconductive web entrained about a guide drum. A magnetic brush developer roll serves to feed toner particles to the latent image recorded on the photoreceptor. A mixing drum agitates and mixes the developer mix. The mixing drum is spaced a greater distance from the photoreceptor than the feed brush. The speed of rotation of the feed brush and that of the mixing drum are different from one another. The Drexler patents disclose two magnetic brushes positioned adjacent to a photoconductive web. The brushes are rotated so that they move in a direction opposed to the direction of movement of the web at the points of contact therewith. The feed brush is rotated at a faster rate than the discharge brush. Moreover, the feed brush is space further from the web than the discharge brush.
It is evident, as exemplified by the foregoing patents, that many approaches have been devised to solve the problems encountered in developing an electrostatic latent image in a high speed electrophotographic printing machine. However, many of the problems associated with high speed development have not been overcome.
Accordingly, it is a primary object of the present invention to improve the development system employed in an electrophotographic printing machine so as to produce high quality copies at high speeds.